The development of earth-abundant and nontoxic catalysts for precision chemical transformations is an endeavor of modern synthesis, and recent years have seen a large effort in replacing precious group 9 and 10 metal catalysts with their lighter base metal congeners. Complementary to this research, many synthetically practical processes that utilize earth-abundant early transition metal or lanthanide catalysts have also been developed. However, due to the oxophilic, electropositive nature of early transition metals, the majority of organometallic catalytic processes that utilize them are redox-neutral. Examples of catalytic processes that rely on early transition metal redox processes are rare and mostly limited to C—C or C—H bond forming reactions, the most notable class being Kulinkovich-type cyclopropanations. Instead of proceeding through oxidative processes, early transition metal catalyzed C—N bond forming reactions typically occur through redox-neutral alkene/alkyne hydroamination and related pathways: the only example of catalytic oxidative C—N bond formation is the utilization of a Zr complex with a redox noninnocent ligand to catalytically couple isonitriles and azides to form carbodiimides.
Polysubstituted pyrroles play a key role in pharmaceuticals, materials, dyes, and natural products, and are often challenging synthetic targets. Various heterocycles have previously been formed from early transition metallacycles, but these reactions are predominantly limited to stoichiometric reactivity.
There is a continuing need in the art for new catalysts and methods for preparing pyrroles.